1. Field of the Invention
The present invention relates to an active matrix display apparatus provided with nonlinear resistance elements, and a method for fabricating the same.
2. Description of the Related Art
With the downsizing of office automation apparatus such as personal computers, demands for flat displays with enhanced functions have increased. In recent years, some types of flat displays have been developed, which include: displays where a voltage is applied between a pair of electrodes sandwiching a display medium having electro-optic characteristics to realize a display; and displays utilizing phenomena such as electro-luminescence, plasma emission, and electrochromic. In particular, a liquid crystal display using liquid crystal as the display medium has found a variety of applications such as watches, calculators, personal computers, and television sets, and such applications have already been put on the market.
With the recent multi-media propagation of information, the displays are increasingly required to have enhanced functions such as high resolution, high contrast, full color, and power save drive. One example of displays that satisfies the above requirements is an active matrix liquid crystal display where active elements are provided for respective pixels. As the active elements, thin film transistors (hereinafter, referred to as TFTs) and thin film diodes (hereinafter, referred to as TFDs) are now under vigorous development. TFDs have a simpler structure and the number of masks required in the fabrication process is smaller, compared with TFTs. Therefore, further cost reduction of displays using TFDs is expected in the future. As one type of TFDs, Japanese Patent Publication No. 61-32674 discloses a MIM (Metal-Insulator-Metal) element having a nonlinear resistance layer made of tantalum oxide. Liquid crystal displays including such MIM elements are now put on the market.
Hereinbelow, a liquid crystal display including conventional MIM elements made of tantalum oxide will be described with reference to FIGS. 12A and 12B. The liquid crystal display includes a MIM substrate having MIM elements as the active elements and a counter substrate. FIG. 12A is a partial plan view showing one pixel portion of the MIM substrate on which a MIM element 207 is disposed, and FIG. 12B is a sectional view taken along line 12B-12B of FIG. 12A.
As shown in FIGS. 12A and 12B, a scanning line 206 made of tantalum and a lower electrode (scanning electrode) 202 branching from the scanning line 206 are formed on a glass substrate 201. Over the lower electrode 202 is formed a nonlinear resistance layer 203 made of tantalum oxide, and an upper electrode 204 made of chrome or the like is formed on the nonlinear resistance layer 203. The lower electrode 202, the nonlinear resistance layer 203, and the upper electrode 204 constitute the MIM elements 207. Thereafter, a pixel electrode 205 composed of a transparent conductive film of ITO (Indium Tin Oxide) and the like is formed so as to overlap the upper electrode 204 for the electrical connection with the MIM element 207. The nonlinear resistance layer 203 is obtained by anodic oxidation of the lower electrode 202.
In order to obtain a high-quality liquid crystal display including the MIM elements as the switching elements, the following conditions are required: (1) The capacitance of the MIM element should be sufficiently smaller than the capacitance of liquid crystal; and (2) the current (I)--voltage (V) characteristic of the MIM element should be abrupt and the current ON/OFF ratio should be large. In the conventional MIM element made of tantalum oxide, the nonlinear resistance layer is formed by anodic oxidation as mentioned above, which is advantageous in forming a uniform film. However, this is disadvantageous in that the current ON/OFF ratio is not so large. The I-V characteristic of the MIM element made of tantalum oxide is considered to follow the Poole-Frenkel formula as shown in formula (1): ##EQU1##
n: carrier density
.mu.: mobility
q: carrier charge amount
s: area of element
d: film thickness
.phi.: trap depth
.epsilon.: dielectric constant of insulating film
.epsilon..sub.0 : dielectric constant of vacuum.
The value of .beta. is an important parameter for determining the ON/OFF ratio. Since tantalum oxide has a high dielectric constant .epsilon. of 23 to 25, the value of .beta. for the tantalum oxide is small compared with that for a film with a low dielectric constant. The value of .beta. for a general MIM element made of tantalum oxide is approximately 3. When the ratio of the current obtained at the application of 20 V to the current obtained at the application of 5 V is used as the ON/OFF ratio, the value of I.sub.200 /I.sub.SV (I.sub.20V represents the current at the application of 20 V and I.sub.SV at the application of 5 V) for the MIM element made of tantalum oxide is as low as the order of 10.sup.3. A high ON/OFF ratio is thus not obtainable. This high dielectric constant also results in increasing the ratio of the capacitance of the nonlinear resistance element to the capacitance of the liquid crystal. This prevents the MIM element from being efficiently applied with voltage.
The object of the present invention is to provide a display apparatus including MIM elements with a large current ON/OFF ratio and a small capacitance so as to achieve high-resolution and high-contrast display.